Measurement of the curvature of a surface for the purpose of determining stress levels therein has particular importance to microelectronic processing where thin films of dissimilar materials are emplaced atop each other. In some cases stress will build up because of the processing conditions, thermal expansion, or the mismatch of various characteristics of the different materials. This stress can cause delaminations and stress-related breakage of metal interconnects. Measurement of the curvature of the surface of these structures and test structures can be related to the stress levels in the structure according to the guidance provided in the literature, one reference being an article by P. A. Flinn, D. S. Gardner, and W. D. Nix "Measurement and Interpretation of Stress in Aluminum-Based Metallization as a Function of Thermal History" IEEE Trans. On Electron Devices, Vol. ED-34, No. 3, March 1987, pp. 689-699.
To date these measurements of the curvature of the surface have been made by rather complex machines that employ a single laser beam to scan the surface or a pair of laser beams. The scanning technique is subject to error due to vibrations of the system as the beam scans across the surface thereby introducing possible errors from point to point. Also, the difficult alignment and mechanical requirements of these systems make them difficult to use remotely, for instance they are unable to operate through a viewport in a microelectronic processing chamber. This undesirably adds another step to manufacturing process if one needs to measure stress levels.